Method of making a heat pipe

ABSTRACT

A heat pipe wick structure including a homogenous central wick is fabricated by forming a plurality of laterally disposed longitudinal pleats in a sheet of wire screen, and compressing the formed pleats laterally together and inserting the same longitudinally into a tubular container of a heat pipe. The plates can be formed in a laterally central portion of the sheet with a flat screen portion on each side thereof, and the side portions are turned back over opposite sides of the formed central wick to serve as wall screens in the tubular container. Reservoir screens having a generally U-shaped cross section normally engaged by the formed pleats can be additionally installed on both side of the central wick in the tubular container.

United States Patent 1191 Waters M lMarch '20, 1973 METHOD OF MAKING A HEAT PIPE Primary Examiner-Charles W. Lanham .75 Inventor: Elmer Dale Waters, Richland, Assistant EMMWFD- Reiley w Attorney-Walter J. Jason et al. [73] Assignee: Mcbonnell l ouglas Corporation, ['57] ABSTRACT Santa Monica, Calif. r I V r A heat pipe wick structure including a homogenous Filed: p 20,1971 central wick is fabricated by forming a plurality of 21 APPL 132 03 laterally disposed longitudinal pleats in a sheet of wire screen, and compressing the formed pleats laterally together and inserting the same longitudinally into a (g1. ..29/157.3ll;2,1ld6gg tubular container of a heat pipe. The plates can be 3 formed in a laterally central portion of the sheet with F [58] of Search 29/157 3 165/105 a flat screen pomon on each slde thereof, and the s1de [56] References Cited portions are turned back over opposite sides of the formed central wick to serve as wall screens in the tu- UNITED STATES PATENTS bular container. Reservoir screens having a generally 1 U-shaped cross section normally engaged by the 3,490,718 1 1970 V ..165 105X 3,498,369 351970 gg 29/157/3RX formed pleats can be add1t1onally mstalled on both 3,620,298 11 /1971 Somerville et a]... 165/105 side of the central Wick in the tubular container- 3,677,329 7/1972 Kirkpatrick ..l65/l05 3,681,843 8/1972 Arcella et al ..29/157.3 R x I 7 Clams, 4 Drawing Flgures BACKGROUND OF THE INVENTION My present invention pertains generally to the field of heat pipes and, more particularly, to a novel heat pipe wick and method of making the same.

A great deal of trouble has been encountered in making and installing a porous central wick in a heat pipe. The earlier porous central wicks were fabricated form foamed metals in a generally rectangular cross sec tional configuration and longitudinally installed in the tubular container of a heat pipe. For relatively long heat pipes, however, the producers of porous (foamed) metals have not been able to make long and uniform wick structures which can be feasibly installed in a heat pipe. The foamed metals are quite fragile and, moreover, their effective porosity or permeability to liquid flow is frequently not as good as desired or required. Of course, it is impossible to install a foamed metal wick in an angularly formed or bent heat pipe, or to form or bend thetubular container having a foamed metal wick installed therein without seriously damaging the wick. A better and more durable porous central wick was obviously desirable and clearly needed for heat pipes.

SUMMARY OF THE INVENTION Briefly, and in general terms, my invention is preferably accomplished by providing a heat pipe wick structure including a homogeneous central wick which is fabricated by forming a plurality of laterally disposed longitudinal pleats in a sheet of wire screen of predetermined length and width, and compressing the formed pleats laterally together and'inserting the same longitudinally into an open end of the tubular container of a heat pipe. The central wick is secured diametrically in position by the elastic expansion of the formed pleats in the tubular container and consequent engagement therewith. The pleats are arranged in a stack providing a plurality of longitudinal passageways with small diameter pores or openings. Compression of the stack reduces the pore diameter of the passageways.

The plurality of pleats can be formed in a laterally central portion of a sheet of wire screen with a flat screen portion on each side of the central portion. The side portions can be turned back over opposite sides of the formed central wick and, after installation of the central wick and turned side portions in the tubular container of a heat pipe, the turned side portions are .positioned therein as wall screens for conveying and spreading liquid to the tubular container wall and any circumferential or longitudinal grooves thereof.

Reservoir screens to take up excess liquid and prevent possible blockage of a cooler (condenser) portionof the heat pipe can also be installed therein. The reservoir screens are preferably formed from respective sheets of wire screen of predetermined length and width, and are longitudinally installed on both sides of the formed central wick. Each reservoir. screen preferably has a generally U-shaped cross section wherein the open ends are engaged by the compressed pleats of the central wick and are thus held in a fixed position in the heat pipe.

BRIEF DESCRIPTION OF THE DRAWING My invention will be more fully understood, and other features and advantages thereof will become apparent, from the following description of an exemplary embodiment and method of making the invention. The description is to be taken in conjunction with the accompanying drawing, in which:

FIG. 1 is an elevational end viewof a heat pipe having a sintered, homogeneous central wick;

FIG. 2 is an elevational end view of a wick being schematically formed according to this invention for installation in thetubular container of a heat pipe;

FIG. 3 is an elevational end view of a heat pipe ineluding a wick structure fabricated in accordance with my invention; and

FIG. 4 is a fragmentary perspective view, shown partially in schematic and diagrammatic form, of illustrative apparatus for fabricating the wick structure and installing it in the tubular container of a heat pipe.

DESCRIPTION OF THE PRESENT EMBODIMENT AND METHOD In the following description of an exemplary embodiment and method of making my invention, some specific dimensions and types of materials are disclosed. It is to be understood, of course, that such dimensions and types of materials are given as examples only and are not intended to limit the scope of this invention in any manner.

FIG. 1 is an elevational end view of an open (uncapped) heat pipe 10 including an elongated tubular container 12 and a central (diametrical) homogeneous wick 14. The container 12 has a generally circular cross section, and the wick 14 has a generally rectangular cross section with slightly rounded ends. The container 12 is made of a suitable metallic tube having a relatively thin wall thickness. Thewiclc 14 is fabricated by stacking many layers of fine wire cloth in a stack and then vacuum sintering the stack into an integral assembly which is press-fitted into the container 12. A dense homogeneous wick of fine pore size capable of pumping against high gravity heads is thus produced. The assembly of wire cloth layers and sintering processes are, however, very time-consuming and another method of making a porous, central wick structure was developed. I

FIG. 2 is an elevational end view of a wick 16 being schematically formed according to this invention. The basic construction and features of the wick 16 are illustrated in this figure. The wick 16 is constructed in a long continuous length of fine woven wire screen 18 folded lengthwise back and forth along its width into a vertical stack or core 20 of horizontal pleats or convolutions 22. The lower and upperend portions S and y of the screen 18 are formed into semicircular cylindrical wall screens as indicated by the arrows 24 and 26,

and the entire structure is inserted into an elongated tubular container preferably of a circular cross sectional chamber. The stack 20 is vertically compressed together as indicated by arrows 28 and 30 as it is in serted and installed in the tubular container. It is noted that the end portions 8 and y can be omitted from the wick 16, particularly when afine screw thread (not shown) has been tapped in the internal wall surface of the tubular container to spread the working fluid (liquid) therein. If only a number of formed pleats is desired, these can be cut out of the stack 20.

It can be easily seen that the wick 16 is readily selfsupporting within a heat pipe extrusion (tubular container) and creates its own spring force, both in the stack 20 and in the end portions S and y, to ensure good diametrical and circumferential wall contacts, respectively. The folded screen wick structure clearly has good stability and is not fragile. It can be tailored to achieve whatever pore size desired. When the screen 18 was folded a number of times into the stack 20, small individual passageways 32 are created which act as longitudinal pores or flow passages to give high axial conductance of the working fluid (liquid). If a smaller pore size is needed for pumping or refilling against a gravity head, then more folds of the screen 18 are provided and the stack 20 pressed more firmly together into the tubular container. Nevertheless, the wick 16 (central stack 20) has an effective permeability to liquid flow of the order of 100 times that for an equivalent multilayered sintered wick 14 (FIG. 1). The wick 16 can also be fabricated very inexpensively as compared to the sintered wick 14. While the stack 20 preferably has a generally rectangular cross section, it can be of a different cross sectional configuration such as an oval one or an non-uniform one wherein the pleats 22 are varied in their horizontal widths vertically along the stack as may be desired or required.

FIG. 3 is an elevational end view of a heat pipe 34 including a tubular container 36 having wick 16 installed therein together with two excess liquid reservoir screens 38 and 40. The wick 16 and reservoir screens 38 and 40 preferably extend substantially over the full internal length of the tubular container 36 which is hermetically closed (sealed) after it has been properly charged with a predetermined amount of a suitable working fluid. The reservoir screens 38 and 40 are provided to take up excess liquid rather than have it a'ccumulate at the cooler (condenser) end portion of the heat pipe 34 and block up such portion thereof. Thus, the reservoir screens 38 and 40 provide static storage spaces to accommodate any volume increases of the working fluid. These screens 38 and 40 can also help maintain the semicircular end portions S and y (wall screens) in firm contact with the internal wall-surface of the tubular container 36 so that the wall screens can better bring liquid to the container wall surface and/or any circumferential screw thread (helical) grooves or longitudinal grooves therein. Of course, the reservoir screens 38 and 40 can be radially shorter and need not contact the wall screens at all.

The wick l6 and the reservoir screens 38 and 40 are quite flexible and this allows the heat pipe 34 to be angularly bent or otherwise formed essentially without damage in any respect. Of course, the central portion (stack 20) of wick 16 retains its high porosity without any damage since it is also flexible but not fragile. Illustratively, for a '6 inch pipe with a 0.42 inch inner diameter, the stack 20 of wick 16 can have a lateral thickness (pleat or convolution width) of 0.1 to 0.12

inch and a pleat spacing gap (curved end portions or' passages) of approximately 0.004 inch, for example. The wick l6 and the reservoir screens 38 and 40 can be fabricated of either a 100 mesh X 0.003 inch diameter wire screen or a 200 mesh X 0.002 inch diameter wire screen. The working fluid can be, for example, anhydrous ammonia. Of course, various other working fluids including water can be used as may be desired or required.

FIG. 4 is a fragmentary perspective view, shown partially in simplified and diagrammatic form, of exemplary apparatus for fabricating the wick 16 and screens 38 and 40, and assembling them together for direct installation in the tubular container 36 (FIG. 3). It is noted that the elevational end view of wick 16 and screens 38 and 40 as shown in FIG. 3 is of one end of the heat pipe 34 whereas the generally similar end view of such elements as shown in FIG. 4- is of the other end (rotated 90) of the heat pipe. The wick 16 is made by passing fine woven wire screen material 42 through, for example, four successive sets of forming rolls 44, 46, 48 and 50. The first three sets of rolls 44, 46 and 48 are preferably followed by respective sets of combs 52, 54 and 56. The sets of rolls 44, 46 and 48, and their respective sets of combs 52, 54 and 56 have pleat forming teeth profile angles of 120, 90 and 60, for example. The last set of rolls 50 has a pleat forming teeth profile angle of, for example, 30. Thus, as the screen material 42 is passed from one set of rolls (and combs) to the next, the pleated shape is progressively formed until it becomes ready for insertion into the tubular container 36. The last set of rolls 50 need. not be followed by a set of combs because of the fineness of the pleats formed by that time.

Simultaneously with the formation of the pleated material 42, reservoir screen materials 58 and 60 are being formed into U-shaped reservoir screens 38 and 40 by respective sets of rolls 62 and 64. The unpleated end portions or side flanges S and y of the screen material 42 can be turned over by a wick turning guide 66 to form such side flanges to the inner diameter of tubular container 36 for wall screens. The guide 66 includes lower and upper sections 68 and 70 which are rigidly secured to fixed structure or are fixedly mounted within a suitable housing (not shown) attached to fixed structure. The guide 66 is preferably tapered conically to flare radially outwards a small amount from front to back in order to facilitate insertion and turning of the pleated screen material 42. The reservoir screens 38 and 40 are, of course, inserted into the guide 66 at the same time as the screen material 42 which is formed into a wick 16 including central stack 20 and wall screens S and y. 7

It is noted that in the end view configuration shown in FIG. 4, the upper reservoir screen 38 is laterally spread very slightly wider at its open end (cross sectionally viewed) than is the lower reservoir screen 4.0 and both screens are, in this instance, illustratively shown radially shorter than as indicated in the other end view of FIG. 3. These are very minor and inconsequential differences since the main purpose of the reservoir screens 38 and 40 is to provide static storage spaces to take up excess liquid that would otherwise accumulate in the vapor spaces at the condenser end of the (closed) tubular container 36. The pleats of the central stack 20 merely engage the open ends of the reservoir screens 38 and 40 to secure them structurally.

For greater structural stability, the reservoir screens 38 and 40 are preferably secured at a middle section of radial open ends of the screens 38 and 40 can be secured to sections other than the middle section of central stack 20, either both to the same section or separately to respectively different sections of the central stack. In fact, if the reservoir screens 38 and 40 were structurally secured by other means than by the (pleats of) central stack 20, the open ends of the screens 38 and 40 can be closed and they can be positioned in any location in the spaces between the central stack and the wall screens S and y.

Referring further to FIG. 3, the radially inner open ends of the reservoir screens 38. and 40 are secured between pleats at each side of a middle section of the central stack 20.-The radially open ends of the screens 38 and 40 can be spaced as close together as possible so long as they can be structurally secured firmly by the pleats of the stack 20. The radially open ends of the screens 38 and 40 should not, however, be secured excessively spread apart by a middle section (of the stack 20) which is morethan, for example, about one-sixteenth of the stacks diametrical length.

Liquid normally enters the static storage space of a reservoir screen through its longitudinally (passageway) open end. In FIG. 3, this is perpendicular to the plane of the paper into the passageway area generally enclosed by a reservoir screen. While the longitudinally open end of a reservoir screen normally abuts against the plane of the closing end cap, there are abutting gaps which are large compared to the screen pores to provide preferred (lower resistance) passage of liquid therethrough and entry into the reservoir screen through its longitudinally open end. Of course, where the cooler (condenser) portion of the heat pipe 34 is located in a longitudinally middle part of the tubular container 36, excess liquid from the liquid in the stack 20 enters the reservoir screens 38 and 40 through the radially open ends thereof. In this instance, the reservoir screens 38 and 40 are preferably secured to the middle section of stack 20, where the liquid normally tends to concentrate.

It can be seen from FIG. 3 that the reservoir screens 38 and 40, together with the stack 20, divide the tubular container 36 cross sectionally into six passageways with three on each side of the diametrical stack. The passageways of the screens 38 and 40 must have a greater capillary pumping action than their adjacent passageways (areas exterior thereto) in order to fill with liquid first. Capillary pumping pressure of a passageway is broadly dependent upon its effective capillary pumping radius or, very generally, proportional to the sum of the reciprocals of the cross sectional width and length of the passageway. Thus, the radially open ends of the screens 38 and 40 cannotbe spread too far apart at the stack 20 since the screens effective capillary pumping radii are then excessively increased, suchthat their pumping pressures would be so reduced that they would not be filled first with surplus liquid, and blockage of the condenser end may occur.

The reservoir screens 38 and 40 preferably extend over the full internal length of the heat pipe 34. The screens 38 and 40 can, however, extend over the entire pipe 34 length except for the evaporator region thereof, or the screens can cover only the condenser region of the heat pipe. The reservoir screens 38 and 40 are preferably provided in the heat pipe 34 where it is to be used in a zero gravity environment, and are preferably omitted where it is to be used in a gravityv field which normally serves to return liquid to the evaporator region. The wall screens S 'and y and/or suitable liquid spreadinggrooves in the internal wall surface of the tubular container 36 are desirably used in both zero and normal gravity environments to provide proper spreading and resupply of liquid to the walls in the heat pipe 34 under dynamic operating conditions.

In summary, a simple and efficient method has been disclosed for making a wick structure in long lengths and assembling it in a heat pipe without requiring expensive tooling. The wick structure can be fabricated without the need for special furnaces employing relatively difficult sintering processes. A wick structure is produced which can be tailored to meet the desired permeability requirements of any particular application. The wick structure can be made with a high porosity (low effective density) and yet not be fragile. The wick structure is also highly flexible and allows a heat pipe containing it to be angularly bent or otherwise formed to a considerable degree without damage to either part. i

While an exemplary embodiment and method of my invention have been described above and shown in the accompanying drawing, it is to be understood that the particular embodiment and method described are merely illustrative of, and not restrictive on, the broad invention and that various changes in design, structure and arrangement may be made in the invention without departing from the spirit and scope of the appended claims.

I claim: l. A method of making a wiclt structure for a heat pipe, which comprises the steps of:

forming a plurality of laterally disposed longitudinal pleats in a first sheet of wire screen of Predetermined length and width; and compressing said formed pleats laterally together and inserting the same longitudinally into an open end of a tubular container, said pleats being secured in position by elastic expansion thereof in said tubular container and consequent engagement therewith.

2. The invention as defined in claim 1 wherein said plurality of laterally disposed longitudinal pleats are formed in a laterally central portion'of said first sheet with a flat screen portion on each side thereof, and further comprising the step of cutting said first sheet longitudinally at a selected lateral width within said central portion to provide a strip including a number of formed pleats from said plurality of pleats, said number of formed pleats of said strip being compressed together and inserted longitudinally into the open end of said tubular container.

3. The invention as defined in claim 2 further comprising the steps of forming at least one reservoir screen from a second sheet of wire screen of predetermined length and width, and installing said formed reservoir screen longitudinally in a fixed position in said tubular container.

4. The invention as defined in claim 3 wherein said second sheet is longitudinally formed into a reservoir screen having a generally U-shaped cross section, and said formed reservoir screen is fixedly installed in said tubular container with the U-shaped cross section open ends of said reservoir screen engaged by the compressed pleats of said strip in said tubular container.

5. The invention as defined in claim 1 wherein said plurality of laterally disposed longitudinal pleats are formed in a laterally central portion of said first sheet.

setting the same with said turned side portions longitudinally into the open end of said tubular container, said turned side portions being positioned therein as wall screens.

6. The invention as defined in claim 5 furthercomprising the steps of forming at least one reservoir screen from a second sheet of wire screen of predetermined length and width, and installing said formed reservoir screen longitudinally in a fixed position in said tubular container between said central portion of formed pleats and one of said wall screens.

7. The invention as defined in claim 6 wherein said second sheet is longitudinally formed into a reservoir screen having a generally U-shaped cross section, and said formed reservoir screen is fixedly installed in said tubular container with the U-shaped cross section open ends of said reservoir screen engaged by the compressed pleats of said strip in said tubular container. 

1. A method of making a wick structure for a heat pipe, which comprises the steps of: forming a plurality of laterally disposed longitudinal pleats in a first sheet of wire screen of Predetermined length and width; and compressing said formed pleats laterally together and inserting the same longitudinally into an open end of a tubular container, said pleats being secured in position by elastic expansion thereof in said tubular container and consequent engagement therewith.
 2. The invention as defined in claim 1 wherein said plurality of laterally disposed longitudinal pleats are formed in a laterally central portion of said first sheet with a flat screen portion on each side thereof, and further comprising the step of cutting said first sheet longitudinally at a selected lateral width within said central portion to provide a strip including a number of formed pleats from said plurality of pleats, said number of formed pleats of said strip being compressed together and inserted longitudinally into the open end of said tubular container.
 3. The invention as defined in claim 2 further comprising the steps of forming at least one reservoir screen from a second sheet of wire screen of predetermined length and width, and installing said formed reservoir screen longitudinally in a fixed position in said tubular container.
 4. The invention as defined in claim 3 wherein said second sheet is longitudinally formed into a reservoir screen having a generally U-shaped cross section, and said formed reservoir screen is fixedly installed in said tubular container with the U-shaped cross section open ends of said reservoir screen engaged by the compressed pleats of said strip in said tubular container.
 5. The invention as defined in claim 1 wherein said plurality of laterally disposed longitudinal pleats are formed in a laterally central portion of said first sheet with a flat screen portion of predetermined width on each side thereof, and further comprising the step of turning said side portions back over opposite sides of said central portion of formed pleats prior to compressing said formed pleats laterally together and inserting the same with said turned side portions longitudinally into the open end of said tubular container, said turned side portions being positioned therein as wall screens.
 6. The invention as defined in claim 5 further comprising the steps of forming at least one reservoir screen from a second sheet of wire screen of predetermined length and width, and installing said formed reservoir screen longitudinally in a fixed position in said tubular container between said central portion of formed pleats and one of said wall screens.
 7. The invention as defined in claim 6 wherein said second sheet is longitudinally formed into a reservoir screen having a generally U-shaped cross section, and said formed reservoir screen is fixedly installed in said tubular container with the U-shaped cross section open ends of said reservoir screen engaged by the compressed pleats of said strip in said tubular container. 